Narrow Results

The contribution of depolarizing electric fields and internal stress fields to the thermodynamics of phase transitions in a ferroelectric–dielectric composite material has been found. This contribution leads to renormalization $\alpha$ and $\beta$ coefficients in the decomposition into a series of the order parameters for thermodynamic potential. It has been established that electric fields always decrease the phase transition temperature. The internal stresses can both increase or decrease the temperature of a first-order phase transition and the width of temperature hysteresis. In the case of the first-order transitions, internal stresses can cause a change in the order of phase transition with appropriate material parameters.

Creep experiments were conducted on Cu–8.5 at.% Al alloy in the intermediate temperature range, 673–873 K, corresponding to 0.46–0.72 T_m, where T_m is the absolute melting temperature. The present analysis reveals the presence of two distinct deformation regions (climb and viscous glide) in the plot of log versus log σ. The implications of these results on the transition from power-law to exponential creep regime are examined. The results indicated that the rate-controlling mechanism for creep is the obstacle-controlled dislocation glide. A phenomenological model is proposed, which assumes that cell boundaries with sub-grains act as sources and obstacles to gliding dislocations.

We report the experimental observations of large spatially disk-shaped patterns in an iron (Fe) film system deposited on silicone oil surfaces by a DC-magnetron sputtering method. These disk patterns form spontaneously during deposition and grow successively in vacuum condition after deposition. Their nucleation, growth and evolution are strongly dependent on the sputtering power, deposition period and growth time. The experiment indicates that they may result from the spontaneous organization and gathering of the Fe atoms and atomic clusters driven by the internal stress.

Various metal film systems, deposited on liquid (silicone oil) substrates by thermal evaporating and DC-magnetron sputtering methods, have been successfully fabricated and the stress relief mechanisms are systematically studied by analyzing the characteristic surface morphologies. The experiment shows that the evaporating metal films can move on silicone oil surfaces freely due to the nearly zero adhesion of solid–liquid interface, which results in spontaneous formation of ordered surface patterns with a characteristic sandwiched structure driven by the internal stress. For the sputtering metal film system, however, the top surface of silicone oil can be modified to form an elastomeric polymer layer on the liquid substrate during deposition. Subsequent cooling of the system creates a higher compressive stress in the film, which is relieved by buckling of the film to form periodic wavy structures because the adhesion of solid–elastomer interface is quite strong.

As the key components of grating-based X-ray phase contrast imaging, absorption gratings are essential to be fabricated. In fact, the internal stress is one of the critical issues for the application of the electroplated gold deposit as the absorption metal for absorption gratings. It is common that high internal stress levels can cause the deposit cracking, blistering and peeling away from the substrate material. This study investigates the effect of current density on the internal stress by the bent strip method. The surface morphologies of gold deposits are examined using scanning electron microscopy (SEM) and the crystal structure of the electroplated deposit is analyzed by X-ray diffraction (XRD). The change of current density reverses the internal stress of the deposits from compressive to tensile. The value of deposit stress can be near zero by optimizing the current density.

In this paper, we present the spontaneous formation of an unusual type of ordered structures in thin metal film systems deposited on silicon oil surfaces by a thermal evaporation method. These structures are composed of a large number of parallel rectangular-shaped domains and exhibit anti-symmetric characteristic. The experiment shows that thin solid films can move freely on liquid surfaces due to the near-zero adhesion of the solid–liquid interface, resulting in superposition of the films and formation of the ordered structures. Compressive stress gradient in the films is shown to be the driving force for self-organization. All characteristics of the ordered structures are discussed in detail based on special mechanical behaviors of these near-free sustained films.

The growth mechanism and characteristic ordered patterns of iron (Fe) films deposited on the silicone oil surfaces by a DC-magnetron sputtering method are presented in this paper. It is found that as the film thickness increases, the iron atoms deposited on the oil surface first form compact clusters, then transfer to ramified aggregates and web-shaped structures, and finally form a continuous iron film. The average branch width of the ramified aggregates is about 0.34 μm, which is almost independent of the sputtering power, i.e., the deposition rate. In the continuous iron films, large spatially disk-shaped patterns are observed, which result from spontaneous ordered organization of the iron atoms and atomic clusters driven by the internal stress in this nearly free sustained film system.

In this paper, the internal stress and hardness of Ni–W alloy films with W contents in the range of 0–59 wt% were investigated. The amorphous Ni–W alloy films were electrodeposited with 59 wt% W content and the structure of crystalline alloy films was formed after heat treating. The experimental results showed that heat treating could prepare Ni–W alloy films with lower internal stress compared with low W content alloy films, and the heat treated alloy films still have high hardness. The internal stress and the hardness were about 230 MPa and 1000 Hv, respectively, when the heat treating temperature was 550°C. The relation of internal stress and hardness with the structure of alloy films is discussed.

The formation mechanism and surface evolution of thin silver films deposited on silicone oil substrates by a DC-magnetron sputtering method are reported. As the film thickness increases, the deposited silver atoms first form compact clusters, then transfer to ramified aggregates and finally form a continuous film on the liquid substrate. After deposition, the surface morphology of the silver film is susceptible to evolve successively in the atmosphere condition, resulting in the formation of broad cracks and straight-sided (or worm-like) wrinkles. The evolution behaviors and underlying physical mechanisms of the cracks and wrinkles are presented and discussed in detail.

In this paper, an optical microscopy study of orderly structures, namely bands, which are observed in a nearly free sustained copper (Cu) film system, is presented. The band is composed of a large number of parallel key-formed domains with different width w but nearly uniform length L. This study shows that the morphologies of the Cu films are very susceptible to the deposition rate, i.e. with the increase in the deposition rate f, the bands with rectangular domains first become irregular gradually and then disappear completely. The experiment indicates that the growth mechanism of the orderly patterns can be explained in terms of the relaxation of the internal stress in the films, which is related to the characteristic boundary condition of the films on the liquid substrates and the nearly zero adhesion of the solid–liquid interface.

We report on the buckling morphologies and interfacial properties of silicon nitride films deposited on float glass substrates. The coexistence of straight-sided and telephone cord buckles can be observed in the silicon nitride films after annealing at a high temperature. The straight-sided structure is metastable and can spontaneously evolve into the telephone cord structure accompanied by the increase in the buckle width and height. The geometric parameters of various buckling structures (including the straight blister, telephone cord and their transition state) have been measured by optical microscopy and atomic force microscopy (AFM). The internal stress and interfacial adhesion of the films are evaluated and analyzed based on the continuum elastic theory. It is valid to measure the interfacial properties of thin films by simplifying the telephone cord buckle as a straight-sided structure. This measurement technique is suitable for all the film systems provided that the buckles can form in the film.

Metal (iron and nickel) films have been deposited on soft elastic polydimethylsiloxane (PDMS) substrates by direct current sputtering technique and the impurity induced wrinkling patterns are investigated by using optical microscopy and atomic force microscopy. It is found that the metal films can spontaneously form disordered wrinkles due to the isotropic compressive stress. In the vicinity of film impurities such as extraneous particles, linear defects, cracks and thickness-gradient film edges, the stress field becomes anisotropic owing to symmetry breaking and thus complex wrinkling patterns including straight stripes, herringbones, crossings, labyrinths and their transitions can be observed. The morphological evolutions, structural characteristics and physical mechanisms of the impurity induced wrinkles have been discussed and analyzed based on the continuum elastic theory.

The monolayer diamond film and free-standing diamond/Mo multilayer films were prepared by combining microwave plasma chemical vapor deposition (MPCVD) and double glow plasma surface alloying (DGPSA) techniques. The result shows that the film grain size decreases from 70–80$\mu$m to $\sim 10$–20$\mu$m with the layer number increasing from the monolayer diamond film with a columnar crystal structure to the seven-layer diamond/Mo films, indicating that the Mo interlayer can effectively reduce the grain size. The Mo₂C is formed between the Mo and diamond layers, which can improve the bonding strength. The internal stress of these films depends on the number of Mo interlayers and the Mo₂C content. Moreover, the fracture strength and abrasion ratio of multilayer diamond films are effectively improved by adding the Mo interlayer. The four-layer diamond/Mo film system has the highest fracture strength and the seven-layer film system has the highest abrasion ratio.

The biofilm wrinkle evolution is the growth mechanism by which bacteria regulate their physiological state in response to the environmental change. We use the parameter of surface complexity to describe different wrinkle patterns. The surface complexity is defined that the biofilm surface area contact with the air is divided by the projected area of the biofilm. We find that the biofilm surface complexity variation is positively proportional to the number of spores. Although each wrinkle pattern has various wrinkle thickness and width, surface complexities of some patterns are almost same, which guarantees cells have enough living space. Through the observation of the growth of the damaged biofilm, we further find that the biofilm expansion along the circumferential direction is faster than that along radial direction, which means that the internal stress along the circumferential direction contributes the wrinkle formation. Our work provides a new perspective to study biofilm morphologies, and relates the morphology evolution with phenotypes in the Bacillus subtilis biofilm.

Microstructures fabricated by use of focused ion beam (FIB) direct scanning is useful for microsystems. However, a new phenomenon was found recently by our experimental results. After the FIB bombardment in the defined area, the microstructures were formed under the competence between the ion sputtering induced surface roughening and surface diffusion induced surface smoothing. The process was called self-organized formation. The generated microstructures were characterized by atomic force microscope (AFM). The measured results show that the dimensions of the microstructures were changed with time. In other word, the measured results were different between the samples just processed and that one finished over one week, and the former with smaller dimensions than the latter. The reason maybe due to the thermal effect induced by the FIB bombardment. We called it a natural annealing. The fabricated microstructures were not in stable state due to the internal stress caused by ion sputtering and thermal diffusion. At initial state, it is in dynamic balance. After the natural annealing with several days, the dynamic balance is changed by the thermal diffusion which caused the dimensions of the microstructures varied continuously. In addition, the surface oxidation is also another factor.

Asphalt concrete is a composite material comprised of aggregate skeleton and asphalt cement. Its permanent or viscoplastic (VP) deformation is characterized by time and temperature dependence, memory effect, and pressure sensitivity. To the best knowledge of the authors, there have been no VP models explicitly considering the nonlinear temperature effects in permanent deformation of asphalt concrete. In previous work, the authors developed a triaxial VP model that featured a VP relaxation spectrum and a modulus parameter $E_{\mathrm{\text{vp}}}$,${}_{\infty }$. The spectrum was postulated as a material function representing the time dependence and memory effect, while $E_{\mathrm{\text{vp}}}$,${}_{\infty }$ was only a function of the stress condition to capture the pressure dependence. The objective of this study was to incorporate temperature dependence into the model. It was achieved by introducing the concept of VP shift factor solely to translate the relaxation spectrum. This strategy was inspired by the time–temperature superposition principle in the viscoelastic (VE) theory. The shift factor was identified experimentally at three temperatures different from the model calibration temperature. The VP shift factor function was thus established, and comparison with its VE counterpart showed that the VP response exhibited greater temperature sensitivity. Numerical simulation was conducted at several temperatures to illustrate the nonlinear temperature effects on the VP deformation and relaxation of the internal stress that served as the hardening/softening variable.

In order to study the effect of different recoating methods on the performance of room temperature vulcanized silicone rubber (RTV), old and new coatings of RTV were tested using the row grid method, scanning electron microscopy and water boiling method. It is found that the old coating of RTV produces internal stress as well as swelling in the recoating, and that surface-contaminating particles need to be removed before the recoating. The spraying method of recoating is recommended. This research provides effective technical guidance for the recoating of aged RTV.